1. Field of the Invention
This invention relates to a protecting film for a semiconductor device or the like which must transmit ultraviolet rays (UV). More particularly, the present invention relates to a protecting film for a UV-erasable ROM.
2. Description of the Related Art
A silicon nitride film (hereinafter referred to as an "SiN film") has been used in the past as a protecting film for a UV-curable type ROM. This is because the SiN film can be formed at a temperature below 450.degree. C. by a plasma CVD process and the reaction between an Al wiring and a Si substrate can be restricted. Recently, the improvement of characteristics of this protecting film has been pursued. The SiN film is formed by rendering a reactive gas plasma. Japanese Examined Patent Publication (Kokoku) No. 63-53703 stipulates a composition ratio composition ratio SiN in a SiN film to be from 0.065 to 0.825 when creating a SiN film. This reference reports that when the Si/N composition is within this range, the SiN film is transparent to ultraviolet rays having a wavelength of 300 nm or less. When the Si/N composition is below 0.65, the chemical property of the film becomes deteriorated and when it is greater than 0.825, the film does not transmit ultraviolet rays.
In practice, however, the wavelength of the ultraviolet rays used for erasure in UV-erasable type ROMs is about 254 nm. Japanese Unexamined Patent Publication (Kokai) No. 2-162773 describes that the composition ratio Si/N must be not greater than 0.75 in order for the film to transmit the 254 nm ultraviolet rays.
At the same time, Japanese Unexamined Patent Publication (Kokai) No. 3-129734 points out the difficulty in which an SiN film having the composition ratio Si/N within the range of 0.65 to 0.825 exerts a great stress on the base. Accordingly, this prior art reference is characterized in that the SiN film is not directly formed on the base but is deposited on the base with another insulating layer therebetween.
On the other hand, "Plasma CVD Technology and Its Application" (Applied Physics (Oyo Butsuri), Vol. 50, No. 6, pp. 638-649, 1981) described as follows:
"The plasma CVD film is generally under a non-equilibrium state and has a great internal stress. This internal stress greatly changes depending on the film growth conditions, that is, the SiN ratio and the hydrogen atom (H) concentration. When viewed collectively, however, the factor that affects the internal stress is not the SiN ratio but is rather the concentration of the hydrogen atom, and the compressive stress increases with the decrease of the hydrogen atom concentration."
The literature mentioned above ("Plasma CVD Technology and Its Application") illustrates the relationship between the Si/N ratio and an absorption edge wavelength .lambda..sub.0 in its FIG. 7 and shows a content sample having a high H concentration. It can be understood from this drawing that when the hydrogen content is great even at the same Si/N ratio, the absorption edge wavelength increases. Though the compressive stress can be reduced by increasing the hydrogen atom content, adverse influences are invited with respect to UV transmission. It can be concluded, therefore, that a protecting film which can be used alone as a protecting film for an UV-erasable type ROM does not exist.